Barrel Extension and Headspace

[StoneHendge]

The pin and weld project I am embarking on will involve a chambered LBC blank without a barrel extension. The gunsmith I will likely be working with believes that even though an LBC is meant to used with a 0.125" depth bolt, it can be headspaced for a 0.135" Grendel bolt when installing the extension . Is this correct? It's not really a big deal for me but I'd lean towards the 0.135" if possible, but don't want to miss out on grabbing a decent 0.125"bolt in this environment if it isn't (I know where there are Spikes bolts in stock). My ability to think it through has been impaired by the people in the unit below me celebrating AZ's legalization of marijuana by smoking even more weed. They're on the deck below, it's open windows season and they start at 9am. I feel like I'm back in CO.

[StoneHendge]

Looks like I stumped the Horde!

[ricsmall]

[StoneHendge]

Thanks! And that does make sense. Gunsmith will be drilling the gas port, but I had already panicked and ordered one of the Spikes bolts when this was languishing. I had hoped to do a JP Grendel bolt but they are nowhere to be found. But if one pops up now I know I can grab and always use the Spikes as backup for my current LBC.

And a brilliant thought just came to me - if I give him a piece or 2 of sized brass without the shoulder bumped from my current LBC, he should be able to get the Headspace close so the brass would work in both rifles, right?

[ricsmall]

[StoneHendge]

Sweet - I like that plan even better.

[LRRPF52]

[StoneHendge]

Thanks 52. This is my first foray into having to have an extension installed. The barrels are BHWs so the assumption is that they were chambered with a 0.125" headspace with my assumption based on that the threads cut so that when the barrel extension is torqued to target spec, the headspace will be in the 0.0125" range. Based on your response and Ric's response, my understanding now is that headspace can be adjusted by tweaking the torque (much the way a barrel nut's torque sometimes needs to be tweaked to allow a gas tube to clear it in certain nut/handguard systems). Is this correct or am I off base? I guess shims could also be used.

Screenshot_20201111-091020_copy_225x191.png

https://truesportingarmsllc.com/product/chambered-blank-without-extension-243lbc-1x8-16/

As to the bolt face depth argument, I didn't mean to get into that. I'll only say that bolts of both types are hard to find. My experience with Spikes products has been first rate - the Horde isn't their target market, but I've found their fit, finish and function to be top notch. I don't have any first hand experience with the manufacturers or distributors of the 0.0136" bolts currently available on the world wide web.

[Klem]

Stonehenge,

That is an interesting idea - that headspace can be varied by varying the torque on the barrel extension. As I understand it you need at least 100ft lbs of torque on the extension to keep it from coming loose when it heats up and is hammered during firing. I doubt that leaves much headspace left between 100ft lbs and the often recommended 150ft lbs.

That post on Homegunsmith.com where the guy says there is .0035" in headspace is between hand-tight (no torque) and 140ft lbs. Obviously you cannot stop at say 60ft lbs because it has reached the target headspace. I'm not a machininst and don't know how much longitudinal movement there is between 100 and 150ft lbs but I doubt it will be anything useful at all.